Muscle tissue has a high metabolism; in other words, in order to perform the work of contraction, muscles need to consume energy. In muscles, as in all cells, ATP is the "energy currency" that allows muscle cells to contract. As described in the actin-myosin cross-bridge cycle, dephosphorylation of ATP to ADP transfers energy to the myosin heads, cocking them so they may "walk" along the actin filament. The presence of sufficient ATP is one of the conditions necessary for muscles to function.
Muscle cells obtain ATP through several different mechanisms. A small amount of ATP is available in the cytoplasm when the muscle is in its relaxed state. Once that is depleted, ATP is also available to muscle cells through the activation of creatine phosphate. Creatine phosphate is a molecule present in muscle fibers that acts as an energy reserve. During the seconds' worth of an activity of a muscle contraction, creatine phosphate can donate a phosphate group to convert ADP to ATP for use by the cell. This process is reversible, and during times when the muscle is at rest, ATP "reloads" the creatine phosphate so it is available during the next contraction.
Additional ATP is generated by cellular respiration. Cellular respiration is the process in which cells break down fuel sources such as carbohydrates and lipids in order to capture the energy contained within. During times of rest or light activity, muscles primarily convert fatty acids to energy. This process produces a lot of energy, but it requires a number of biochemical steps and can be too slow to keep up with the amount of ATP needed during times of intense or sustained activity. The steps of these pathways take place in the cell's mitochondria, where a large amount of ATP is produced. Because they require oxygen as an oxidizing agent, they are referred to as aerobic respiration.
An alternative metabolic pathway, glycolysis, can produce ATP very rapidly during times of intense activity. Glycolysis uses glucose as its input and does not require oxygen, so it is referred to as an anaerobic process. Muscle cells store glucose in the form of glycogen, which is rapidly converted back to glucose when it is needed. Anaerobic glycolysis produces lactic acid as a byproduct, which can cause the muscle to fatigue (felt as soreness immediately after, but not days later) after an intense workout.
Skeletal Muscle Fiber Types
There are three types of skeletal muscle fibers, and these types differ in their structures and metabolic processes. Slow-twitch muscle fibers are sometimes called "red" muscle because these tissues are dense with capillaries. The muscle fibers in slow-twitch muscles are rich with mitochondria, which produce ATP to power the cells. These cells also contain myoglobin⎯a protein related to hemoglobin. Like hemoglobin in the blood, myoglobin is a pigment that carries and stores oxygen in muscle tissues and muscle cells. The red pigments in myoglobin give slow-twitch muscles their red color. These muscle fibers can sustain a contraction for a long period of time, but they cannot contract with as much force. Because of the prevalence of mitochondria and myoglobin, slow-twitch muscle fibers have a greater capacity for aerobic respiration. They are sometimes called slow oxidative muscle fibers.
Fast-twitch muscle fibers contract quickly and powerfully, but cannot sustain a contraction for as long a period of time. Some types of fast-twitch muscle fibers contain significantly less myoglobin and fewer mitochondria, and are therefore paler in color than slow-twitch muscles. These "white" fast-twitch muscles are more reliant on energy produced through anaerobic respiration (glycolysis), so they are sometimes called fast glycolytic muscle fibers.
Intermediate muscle fibers are sometimes called fast oxidative−glycolytic muscles. These are fast-twitch muscle fibers, capable of strong, rapid contractions, but with a rich capillary supply and myoglobin presence. Thus, this muscle type can produce ATP through either aerobic or anaerobic means. Some researchers think fast-twitch muscles can be transformed into intermediate muscles through endurance training.
Skeletal Muscle Strength and Endurance
Prolonged and intensive activities lead to fatigue and can result from various factors. After exercise, the body responds to an oxygen debt by maintaining a high ventilatory rate. An individual's level of physical activity affects skeletal muscle tissue in a number of different ways. Muscle strength is dependent on various factors, including the size of the muscle, its cross-sectional area, the strength of the neurological signal, and the amount of leverage the muscle has. Muscles can become stronger with training. Although the number of individual muscle fibers cannot be changed through exercise, individual muscle fibers do grow larger as new myofibrils are added with muscle use. This increases the overall size of the muscle.
Prolonged and intensive activities can lead to muscle fatigue. In rare cases, this fatigue can be caused when the nerve that controls the muscle cannot sustain a sufficient signal. Strength training can increase the motor neurons' ability to generate muscle contractions.
Metabolic fatigue is more common. Metabolic fatigue results when muscle fibers run out of fuel such as ATP, creatine phosphate, and glucose from glycogen. Metabolic fatigue can also result when waste products like lactic acid, an acid compound produced in muscles when glucose breaks down and oxidizes faster than the body can break it down, build up in the muscle fibers. In order to recover from intense exercise, the body needs extra oxygen to replenish ATP, myoglobin stores, creatine phosphate, glycogen stores in the skeletal muscle, and oxidizing or removing the lactic acid. The body responds to this oxygen debt by maintaining a high ventilatory rate after exercise.
Muscles become increasingly resistant to fatigue with endurance training. Endurance training may transform fast-twitch muscle fibers into intermediate muscle fibers that have a greater capacity for aerobic respiration. Endurance training also increases the number of capillaries that supply blood to the muscles, and the number of mitochondria present within individual muscle fibers. Endurance exercise also increases the amount of myoglobin, the protein in muscle cells that stores oxygen, that is present in slow-twitch and intermediate muscle fibers.